Currently, 3rd generation cellular communication systems are being installed to further enhance the communication services provided to mobile phone users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD) technology. In CDMA systems, user separation is obtained by allocating different spreading codes and/or scrambling codes to different users on the same carrier frequency and in the same time intervals. This is in contrast to time division multiple access (TDMA) systems, where user separation is achieved by assigning different time slots to different users. An example of communication systems using these principles is the Universal Mobile Telecommunication System (UMTS™).
In order to provide enhanced communication services, the Long Term Evolution (LTE) version of 3rd generation cellular communication systems is designed to support a variety of different and enhanced services. One such enhanced service is a support of multimedia services. The demand for multimedia services that can be received via mobile phones and other handheld devices is set to grow rapidly over the next few years. Multimedia services, due to the nature of the data content that is to be communicated, require a high bandwidth. The typical and most cost-effective approach in the provision of multimedia services is to ‘broadcast’ the multimedia signals, as opposed to sending the multimedia signals in an unicast (i.e. point-to-point) manner. Typically, tens of channels carrying say, news, movies, sports, etc., may be broadcast simultaneously over a communication network. Further description of LTE, can be found in Sesia, Toufik, Baker: ‘LTE—The UMTS Long Term Evolution; From Theory to Practice’, page 11, publ. by Wiley, 2009.
As radio spectrum is at a premium, spectrally efficient transmission techniques are required in order to provide users with as many broadcast services as possible, thereby providing mobile phone users (subscribers) with the widest choice of services. It is known that broadcast services may be carried over cellular networks, in a similar manner to conventional terrestrial Television/Radio transmissions. Thus, technologies for delivering multimedia broadcast services over cellular systems, such as the evolved Mobile Broadcast and Multicast Service (eMBMS) for the LTE aspect of UMTS™, have been developed over the past few years. In these broadcast cellular systems, the same broadcast signal is transmitted over non-overlapping physical resources on adjacent cells within a conventional cellular system. Consequently, at the wireless subscriber unit, the receiver must be able to detect the broadcast signal from the cell that it is connected to. Notably, this detection needs to be made in the presence of additional, potentially interfering broadcast signals, that are transmitted on the non-overlapping physical resources of adjacent cells.
To improve spectral efficiency, broadcast solutions have also been developed for cellular systems in which the same broadcast signal is transmitted by multiple cells, but using the same (i.e. overlapping) physical resources. In these systems, cells do not cause interference to each other as the transmissions are arranged to be substantially time-coincident, and hence capacity is improved for supporting broadcast services. Such systems are sometimes referred to as ‘Single Frequency Networks’, or ‘SFNs’. In SFN systems, a common cell identifier (ID) is used to indicate those (common) cells that are to broadcast the same content at the same time. In the context of the present description, the term “common cell identifier” encompasses any mechanism for specifying SFN operation, which may in some examples encompass use of, say, a single scrambling code.
In 3GPP™ Rel10 a concept 100 of relay nodes is being considered for LTE, as illustrated in FIG. 1. The relay concept 100 involves a deployment of Relay Nodes (RN's) 120 in order to extend radio coverage over a Uu interface 125 to those subscriber communication units (referred to as user equipment (UE) in 3G parlance) 130 that are within the coverage area of the RN 120. Backhaul connectivity for the RN 120 is provided using the LTE radio resource over the Un interface 115. In this manner, the RN 120 is connected over the LTE radio resource to an evolved packet core (EPC) 105 via a communication source base station (referred to as an evolved NodeB (eNodeB) in 3G parlance) that may be referred to as a Donor eNodeB (DeNB) 110. From the perspective of UE 130 within the coverage of the RN 120, the RN 120 appears as a conventional eNodeB. From the perspective of the Donor eNodeB 110 the RN 120 appears somewhat like a UE 130.
The issue of supporting eMBMS over a RN has been raised in (Tdoc R2-103960: ‘Considerations on deployment of both relay and eMBMS’. CMCC, 3GPP TSG-RAN WG2 meeting #70bis, Stockholm, Sweden, 28 Jun.-2 Jul. 2010). In this document a method for extending eMBMS was briefly described as:                ‘Under this architecture, the content synchronization should be guaranteed not only from BM-SC to DeNB, but also from BM-SC to RN. In this case, the eMBMS related data needs to be transmitted to the DeNB firstly, and then be forwarded towards the corresponding RNs before transmitting to the UEs.’        
This extract clearly suggests to those in the art that the Donor eNodeB 110 would first forward eMBMS traffic from the DeNB 110 to the RN 120 using a unicast bearer, although no bearer is specified. Once the RNs 120 have received the eMBMS data then both DeNB's 110 and RN's 120 can transmit the eMBMS data over the single frequency network at the same time, such that UE's 130 can easily combine, at the physical layer, the transmissions received from all eNodeB's and RN's 120 that are within range.
A disadvantage of this approach is that the eMBMS traffic is transmitted twice by the DeNB 110, first in the unicast transmission over the Un interface 115 to the RN 120 and secondly when the DeNB 110 makes the eMBMS broadcast itself over the Uu interface 125. Once the RNs 120 have received the eMBMS data, then both DeNBs 110 and RNs 120 are able to transmit the eMBMS data over the single frequency. All transmissions from the relay node layer (or alternatively, simultaneously from both the relay node layer and the eNodeB layer) should occur at the same time. This ensures that any macro-diverse eMBMS transmissions from multiple DeNBs 110/RNs 120 arrive at the UE 130 with time offsets that fall within the cyclic prefix of the OFDM symbol, thereby simplifying the UE's equalisation process e.g. combining at the physical layer the transmissions from all enodeB's and RN's within range. If there are multiple RNs 120 within the coverage of the DeNB 110 then multiple unicast streams carrying the same information would be necessary. This repeated transmission has the disadvantage that it consumes additional eNB radio resources.
A further potential problem with this proposed mechanism is that propagation delays between each RN 120 and its associated DeNB 110 are likely to be different. Thus, should each RN 120 simply re-broadcast the eMBMS information received from the DeNB 110 as soon as the RN 120 receives it, then due to the propagation delay differences on the Un interfaces 115, the transmissions from multiple RNs 120 could not be guaranteed to arrive at the UE 130 within the cyclic prefix window of the UEs (given that there will also be accumulative propagation delay differences on each of the Uu interfaces). Such a problem occurs, for example, if all transmissions from relay nodes were planned to occur at the same time (for example for multicast broadcast SFN (MBSFN) physical layer combining of relay node transmissions at the UE 130), or if it is desired that RN transmissions be symbol aligned with the transmissions from the DeNB 110, so that combining can be achieved at the UE 130 of both DeNB 110 and RN transmissions.
A further potential problem with this proposed mechanism is that it is important that each RN decodes the eMBMS traffic received from the DeNB as accurately as possible, since it may be re-broadcasting this information to many tens or hundreds of UEs. Thus, a probability of correct detection of the eMBMS signal at the RN needs to be high.
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the problem of supporting broadcast transmissions using relay nodes in a cellular network would be advantageous.